Hot melt pressure sensitive adhesives are compositions that combine the properties of hot melt adhesives with the properties of pressure sensitive adhesives. The hot melts are solids at room temperature, melt upon application of heat, and regain their solid form on cooling. The pressure sensitives are aggressive and permanently tacky at room temperature, and adhere to surfaces by the application of light finger pressure. The combination of these properties provide compositions that melt at elevated temperatures and cool to form a permanently tacky solid coating that adheres on contact. These compositions are most commonly applied to various substrates, such as paper, cloth, metal, and plastic films, which are then converted into tapes and labels for use in the packaging industry, particularly in marking, sealing and bonding applications, or for use in the health and pharmaceutical industry, particularly in bandages or transdermal drug delivery systems.
A good workable hot melt pressure sensitive adhesive must exhibit high cohesive strength at room temperature, low shrinkage on substrates (particularly flexible plastic), retention of pressure sensitive properties during storage and use, and a relatively fluid viscosity at commonly employed application temperatures of 80.degree.-180.degree. C. Although very low molecular weight polymers will yield a hot melt with sufficient fluidity, the resulting adhesives lack cohesive strength. High molecular weight polymers give better cohesive strength, but are too viscous at the common application temperatures to be easily coatable on substrates and must be extended with a high proportion of low molecular weight oils or resins to reduce the viscosity. The addition of low molecular weight oils or resins in turn detracts from the cohesive strength and heat resistance.
To circumvent these problems, low molecular weight polymers have been made with olefinic unsaturation on side chains. The olefinic unsaturation can then be crosslinked by ultra-violet or other actinic radiation to provide sufficient cohesion and tack. Ordinarily, most of the olefinic double bonds that polymerize by free radical reactions are also thermally polymerizable. In this regard, it is known that acrylates, methacrylates, styrenes, vinyl ester, and allyl compounds, for example, are quite susceptible to thermally induced polymerization (see, Principles of Polymerization, Second Edition, George Odian, John Wiley & Sons (1981), page 214). A hot melt adhesive, which is a 100% solid composition, must be heated to temperatures in the range of 80.degree.-180.degree. C. in order to realize a sufficient viscosity for the proper processing and coating of the polymer. When hot melt pressure sensitive adhesives with pendant unsaturation encounter high processing temperatures for long periods of time, the pendant unsaturation begins to crosslink and the adhesives exhibit an increase in viscosity and may gel before they can be irradiated.
For example, U.S. Pat. No. 4,234,662, issued 18 Nov. 1980 to Pastor et. al., teaches a hot melt pressure sensitive adhesive prepared by introducing a predetermined amount of pendant vinyl unsaturation into a saturated polymeric chain that is the base for the adhesive. In the method disclosed in this patent, allyl acrylate or methacrylate is copolymerized with an acrylic monomer and reliance is placed on the difference in reactivity between the acrylic double bond and the allylic double bond to control the level of polymerization, and ultimately, the level of crosslinking. However, in commercial practice, this differential is not sufficient and large scale production frequently leads to excessive polymerization and premature gelation.
U.S. Pat. No. 4,665,106, issued 12 May 1987 to Ohta et al., teaches a radiation curable pressure sensitive adhesive. This adhesive is not a hot melt, but does comprise low molecular weight oligomers prepared by introducing one or more olefinic unsaturated bonds into side chains of the adhesive polymer. The process comprises copolymerizing an acrylic ester compound with a monomer having a carboxyl, hydroxyl, epoxy, or amine functional group, and then reacting that functional group in a non-free radical reaction with another monomer having one or more unsaturated double bonds. Although it could be assumed that this method would eliminate premature gelation during polymerization that may be caused by the presence of allyl groups, nevertheless, excessive crosslinking would still appear to be a problem addressed by the addition of large amounts of chain transfer agents to the adhesive syrup.
Therefore, there is a need for a hot melt pressure sensitive adhesive that has good cohesive and adhesive properties and that will remain thermally stable during processing and coating operations, but at the same time, will be easily cured by actinic radiation after processing or coating onto a substrate.